The invention relates to a method of regenerating feed material of coated grains with an irregular surface, in particular used foundry sands which are covered with a sheath or casing of binder or the like, wherein the sheath or casing is opened up by friction.
The various stages involved in the processing of foundry sands are to be found for example in Giessereilexikon, Berlin 1980.sup.11, pages 675 to 681. An essential aspect in that respect is in particular the regeneration of old sands in order to be able to re-use same to a very substantial extent after a casting operation, in order to save on new sand and to reduce the amounts dumped. For that purpose it is necessary inter alia for the individual grains of sand to be freed from the binders or like impurities which enclose the grains of sand as a sheath or envelope thereon, for which purpose in particular frictional or impact forces are used; the grain surface is cleaned by impingement against a surface or by means of friction by sand grain material. In that situation the binder sheaths which cling to the surface of the grain are removed predominantly by impact or friction, depending on the nature of the method involved.
Thus, for example, German patent specification No 2 408 981 proposes that foundry sand is accelerated by rotation and driven at high speed against a bed of bulk material which serves as an impingement cushion; the grains are broken up and cleaned, under the effect of centrifugal force, due to friction in the bulk material bed. Sand cleaning with a centrifugal wheel and inclinedly disposed impingement plates is disclosed in German laid-open application (DE-OS) No 22 02 311.
German laid-open application (DE-OS) No 24 48 333 provides that the sand is accelerated by means of a pneumatic device against an impingement wall which, for example, extends in a hood-like configuration over a vertical, inclined or horizontal sand guide pipe, at a spacing relative thereto. German patent specification No 2 519 135 also describes a cleaning installation for impact cleaning of granular material by means of a nozzle which is connected to a gas flow source, opposite an impact surface.
German patent specification No 2 233 111 describes a pneumatic regeneration method in which used foundry sand, possibly with new sand mixed therewith, is flung up against an impact bell member or the like, by compressed air. When impact occurs, the binder layers which encase the individual quartz grains split open and the quartz grain can then freshly accept a casing of binder thereon. Quartz grains may suffer damage in that treatment and then occur as dust.
In the case of the methods which provide for acceleration by means of air nozzles, the recommendation is that the air flow should be kept turbulent and that optionally, in addition to a primary flow of sand, there should be a similar secondary flow of sand so that the scouring effect is increased.
Microscopic examination has shown that the convex parts of grains of sand which are treated in that way are admittedly cleaned, but considerable residues of binder still remain clinging in the concave zones. When impact loading occurs, the binder sheathing is not completely detached from the grain but remains clinging thereto precisely in the depressions in which it is relatively firmly and almost form-lockingly engaged; because of their similar size, the grains of sand surrounding same are incapable of penetrating into those depressions and removing the residual binder therein by friction.